1. Field of the Invention
The present invention relates to the etherification of isoolefins, particularly C.sub.4 and C.sub.5 isoolefins, with an alcohol such as methanol, ethanol or mixtures thereof, to produce the corresponding tertiary ethers. More particularly the invention relates to a process wherein a catalytic distillation process is used in the process and wherein the concentration of the alcohol below the catalyst bed is controlled to prevent the concentration of the alcohol from becoming too low (and thus reducing the conversion) or too high (and thus contaminating the ether product).
2. Related Information
The reaction of an alcohol and an olefin and concurrent separation of the reactants from the reaction products by fractional distillation has been practiced for some time. The process is variously described in U.S. Pat. Nos. 4,232,177; 4,307,254; 4,336,407; 4,504,687; 4,987,807; and 5,118,873 all commonly assigned herewith.
Briefly the alcohol and isoolefin are fed to a distillation column reactor having a distillation reaction zone containing a suitable catalyst, such as an acid cation exchange resin, in the form of catalytic distillation structure, and also having a distillation zone containing inert distillation structure. As embodied in the etherification of iC.sub.4.sup.= 's and/or iC.sub.5.sup.= 's with methanol the olefin and an excess of methanol is first fed to a fixed bed reactor wherein most of the olefin is reacted to form the corresponding ether, methyl tertiary butyl ether (MTBE) or tertiary amyl methyl ether (TAME) . The fixed bed reactor is operated at a given pressure such that the reaction mixture is at the boiling point, thereby removing the exothermic heat of reaction by vaporization of the mixture. The fixed bed reactor and process are described more completely in U.S. Pat. No. 4,950,803 which is hereby incorporated by reference.
The effluent from the fixed bed reactor is then fed to the distillation column reactor wherein the remainder of the iC.sub.4.sup.= 's or iC.sub.5.sup.= 's are converted to the ether and the methanol is separated from the ether which is withdrawn as bottoms. The C.sub.4 or C.sub.5 cut generally contains about 10 to 60 per cent olefin, the remainder being inerts which are removed in the overheads from the distillation column reactor.
As noted above in the etherification of olefins with an alcohol there is preferably an excess of the alcohol available in the reactor. This means that there is an excess of methanol in the reaction distillation zone of the distillation column reactor. In the distillation column reactor the alcohol forms a minimum boiling azeotrope with either of the olefins. In the case of C.sub.4 's and methanol the azeotrope is only slightly more volatile than the C.sub.4 's, and therefore the methanol tends to remain in a relatively constant concentration with the C.sub.4 's throughout the column. Because the concentration of the methanol in the C.sub.4 azeotrope is only about 4% (depending upon the composition of the C.sub.4 mixture), it is necessary to operate with a methanol concentration somewhat below 4% to avoid exceeding the azeotrope, in which case the, excess will first accumulate in the column and then leave with the MTBE bottoms.
In the case of the C.sub.5 's, the azeotrope contains about 12 wt% methanol or about 8 wt% ethanol, and the boiling point of the azeotrope is 10 to 15 degrees F. below that of the corresponding C.sub.5 's. Thus, if the net flow of alcohol into the column (allowing for that reacting in the column) is less than the azeotrope concentration in the distillate, the alcohol concentration in the reaction distillation zone will be relatively quite low, about 1%. If the net alcohol flow into the column is higher than the azeotrope, the alcohol concentration will increase (60% has been measured) until alcohol leaves with the ether bottoms product. Neither case is desirable, because at low alcohol concentration the conversion of isoamylene to ether is low, whereas at high alcohol concentrations the ether purity is affected by the presence of the excess alcohol.
In addition to the considerations of azeotopes discussed above, when two or more different alcohols are fed at the same time, other factors affect the operation of the etherification. For example, in the case of etherification of iC.sub.5.sup.= with a mixed methanol/ethanol stream to produce TAME and tertiary amyl ethyl ether (TAEE) , there are two different azeotropes. The first is methanol with the C.sub.5 's which is 12% methanol. The second is the azeotrope between the C.sub.5 's and the ethanol which is 8% ethanol. The different alcohols also react at different rates with the isoolefins, e.g., methanol reacts more rapidly than ethanol with isopentenes.
It as an advantage of the present invention that the alcohol-hydrocarbon azeotrope is maintained throughout essentially all of the reaction distillation zone, maximizing conversion of the reactive olefins.
It is a second advantage of the present invention that the control of the alcohol concentration in the liquid leaving the reaction distillation zone insures an alcohol free ether product.
It is another advantage that a truncated alcohol concentration profile improves controllability of the column.
Finally, it is another advantage that the concentration of either methanol or ethanol is controlled by a single variable to assure adequate alcohol concentration and an alcohol free product.